Flying disc aerodynamics

[halfeye]

There's this thing, where if you make a flying disc spin fast and fly slowly, after flying for a while it will more or less stop, wobble, speed up while apparently losing its spin, and change direction slightly.

Does anybody else remember this, or is it just me?

[gomipile]

Yes, I'm familiar with that behavior. Looking back, it seems like what's happening is an aerodynamic stall, followed by a brief period of unstable near-equilibrium, which randomly breaks into a different glide condition than the initial launch.

[halfeye]

Yes, I'm familiar with that behavior. Looking back, it seems like what's happening is an aerodynamic stall, followed by a brief period of unstable near-equilibrium, which randomly breaks into a different glide condition than the initial launch.

Yes, I agree with all of that I think, it is what happens in the wobble during the stall that I think is interesting.

What seems to me to happen is that rotational energy is turned into directional energy, and i don't understand how that happens.

Is the flexibility of the disk part of the process?

Does the rotation cause the disk to push against the air?

[Radar]

Rotation is there to keep the disc from turning and energy connected to that is not turned into forward motion. The disc getting wobbly means its not exactly parallel to the Earth's surface anymore. If so, there appears a direction easier for it to slide to in the air that is slightly downward, so gravity accelerates it.

[halfeye]

Rotation is there to keep the disc from turning and energy connected to that is not turned into forward motion. The disc getting wobbly means its not exactly parallel to the Earth's surface anymore. If so, there appears a direction easier for it to slide to in the air that is slightly downward, so gravity accelerates it.

Have you actually seen the phenomenon I'm talking about?

[Radar]

Have you actually seen the phenomenon I'm talking about?

To be honest, no. If you have some video recording, it would be interesting to see.

I do know the physics behind it though. Not sure if I were clear in describing it, but it is not possible for the rotational energy to change into forward motion in this situation. Rotation only prevents the disc from tilting (or makes tilt require more effort - like wheels in a bike) and that slows the downfall due to air resistance. This wobble you see is very similar to what happens to a spinning top when it slows down - outside forces want to turn the disc in a direction perpendicular to its current rotation, but due to the high angular momentum it turns in a perpendicular direction instead resulting in a wobble. Eventually the disc stops being horizontal anyway, so there is direction for it to slide down (even a very small downward angle will be enough) without much air resistance.

[halfeye]

To be honest, no. If you have some video recording, it would be interesting to see.

I don't watch YouTube, there may be a video there, I don't know.

I do know the physics behind it though. Not sure if I were clear in describing it, but it is not possible for the rotational energy to change into forward motion in this situation. Rotation only prevents the disc from tilting (or makes tilt require more effort - like wheels in a bike) and that slows the downfall due to air resistance. This wobble you see is very similar to what happens to a spinning top when it slows down - outside forces want to turn the disc in a direction perpendicular to its current rotation, but due to the high angular momentum it turns in a perpendicular direction instead resulting in a wobble. Eventually the disc stops being horizontal anyway, so there is direction for it to slide down (even a very small downward angle will be enough) without much air resistance.

Well something happens. It's relatively fast, and there certainly is acceleration during the wobble. I don't know for sure that the energy comes from the rotation, there's nowhere else for energy to come from, gravity on a glide of 10:1 isn't anything like enough.

When aeroplanes start to spin, usually from a stall, that is a case of forward motion turning into rotation, I don't see why the reverse should be utterly impossible.

[Radar]

Well something happens. It's relatively fast, and there certainly is acceleration during the wobble. I don't know for sure that the energy comes from the rotation, there's nowhere else for energy to come from, gravity on a glide of 10:1 isn't anything like enough.

Gravity should be more than enough and there is really no other option. On a glide of 10:1 you are still looking at around 1m/s^2 of acceleration and air resistance is not much of an issue.

So slowing down rotation results in wobbling motion, which in turn creates a slope. If you have a slope, gravity does its thing.

When aeroplanes start to spin, usually from a stall, that is a case of forward motion turning into rotation, I don't see why the reverse should be utterly impossible.

As I was saying, it is not possible in this particular case. The reason for this is the symmetry. If you have a spinning disc and rule out any effects from gravity, there is no special direction in which the disc could be pushed in the horizontal plane (as it will surely not go up or down except for slowly falling). If all the horizontal directions are equal, there cannot be a process that would exchange rotation for a forward motion - the symmetry has to be broken somehow. If there were such a process by the way, it would also occur when the disc is flying normally - not just when it stops in the air.

Interestingly there is a process that does accelerate a spinning object, but only when it moves forward - Magnus effect, which anyone playing table tennis knows well in practice. In that case the motion itself is what breaks the symmetry and the direction of the force depends on both forward velocity and direction of rotation.


Wind might also add to the motion of a disc, but that would be pretty obvious to notice.

[Rakaydos]

Precession might be in play as well, where the axis of rotation itself rotates.

Also aerodynamic effects based on one side of the disk moving into the wind while the other side of the disk is spinning backward at the same speed the disk itself is moving forward, effectively stationary in the air.

Edit: probably both at once. At low speeds, one side of the disk stalls, and the uneven lift causes rapid precession.

[Radar]

Precession might be in play as well, where the axis of rotation itself rotates.

Also aerodynamic effects based on one side of the disk moving into the wind while the other side of the disk is spinning backward at the same speed the disk itself is moving forward, effectively stationary in the air.

Edit: probably both at once. At low speeds, one side of the disk stalls, and the uneven lift causes rapid precession.

Yes, I think that covers the wobbling motion part.

[halfeye]

Precession might be in play as well, where the axis of rotation itself rotates.

Also aerodynamic effects based on one side of the disk moving into the wind while the other side of the disk is spinning backward at the same speed the disk itself is moving forward, effectively stationary in the air.

Edit: probably both at once. At low speeds, one side of the disk stalls, and the uneven lift causes rapid precession.

Yeah I like that. If you really spin the disk as hard as you can, you probably get one side moving backward with respect to the relative wind.

Yes, I think that covers the wobbling motion part.

However, it doesn't explain the sudden increase in speed or the cessation of the wobbling (there is typically also in my experience a change of direction by ten or more degrees). There is energy in the rotation, if it slows that energy has to go somewhere, and I am pretty sure new matter isn't created.

[Radar]

However, it doesn't explain the sudden increase in speed or the cessation of the wobbling (there is typically also in my experience a change of direction by ten or more degrees). There is energy in the rotation, if it slows that energy has to go somewhere, and I am pretty sure new matter isn't created.

The energy is dissipated as heat due to air resistance. And that speed gain is not all that difficult. Wobbling means that the disk is not exactly horizontal anymore. Once the angle is prominent enough, the disk starts accelerating due to gravity and once it starts going forward again, it stops wobbling so much as the air flowing around it helps in stabilizing it. In a sense, it is similar to stalling - speed is too low, so the disk gets unstable. Once the pitch of the trajectory allows it to gain some speed back, things get stable again.

More importantly, if rotation was somehow converted into forward motion in a way that does not require relative velocity between air and disk, the disk would never really do this wobbly stop to begin with.

[halfeye]

The energy is dissipated as heat due to air resistance.

At five mph? Do me a favour.

And that speed gain is not all that difficult. Wobbling means that the disk is not exactly horizontal anymore. Once the angle is prominent enough, the disk starts accelerating due to gravity and once it starts going forward again, it stops wobbling so much as the air flowing around it helps in stabilizing it. In a sense, it is similar to stalling - speed is too low, so the disk gets unstable. Once the pitch of the trajectory allows it to gain some speed back, things get stable again.

It slows because it runs out of forward velocity. Stalling is often a problem for aircraft.

More importantly, if rotation was somehow converted into forward motion in a way that does not require relative velocity between air and disk, the disk would never really do this wobbly stop to begin with.

I never said that there was no relative velocity between air and disk. I quite clearly wrote that the disk was spinning. There was a drop of about three inches (could have been 2 1/2, wasn't fully 4 (with a ten or so inch disk)), while the disk wobbled, then it shot more or less forward at a somewhat predictable angle, decelerating mildly as it went.

[Radar]

At five mph? Do me a favour.

Two types of motion: forward motion and rotation. Both slow down due to air resistance and considering the surface area to mass ratio it is quite significant - if it were not, the disk would not fly the way it does. 5 mph of forward motion does not indicate how fast the disk is spinning and by extension how fast is the energy of rotation decreasing. And there is no other phenomenon that actually could dissipate the energy in the described situation.

It slows because it runs out of forward velocity. Stalling is often a problem for aircraft.

Slowing down (in terms of straight motion) and running out of forward velocity are synonymous and caused by friction with the air. If in the phrase "slows down" you meant the rotation than it is not quite like that as those two types of motion (straight flight and rotation) are mostly decoupled - any interaction between the two is indirect. Air flow around the disk resulting from forward motion stabilizes the axis of rotation and the rotation itself fixes the disk in a horizontal position which allows for gliding and prevents a tumbling motion.

I never said that there was no relative velocity between air and disk. I quite clearly wrote that the disk was spinning. There was a drop of about three inches (could have been 2 1/2, wasn't fully 4 (with a ten or so inch disk)), while the disk wobbled, then it shot more or less forward at a somewhat predictable angle, decelerating mildly as it went.

Again it is important to be clear what kind of motion we talk about - maybe I was not. Rotation is one thing at it is obvious from the initial description of the problem that it is still there. On the other hand, you wrote that the forward motion stops or nearly stops right before you get that wobbling and after that the speed picks up. If there was some hypothetical process that would convert rotation to forward motion without relying on already existing forward velocity (and there is none - such symmetry breaking would have far wider consequences), than it would work all the time and you would never get that wobbling part of motion with next to no forward velocity - the motion would be smooth.

[georgie_leech]

Two types of motion: forward motion and rotation. Both slow down due to air resistance and considering the surface area to mass ratio it is quite significant - if it were not, the disk would not fly the way it does. 5 mph of forward motion does not indicate how fast the disk is spinning and by extension how fast is the energy of rotation decreasing. And there is no other phenomenon that actually could dissipate the energy in the described situation.

Maybe I'm missing something, but couldn't energy, you know, be transferred to the surrounding air? As in, rather than being lost as heat, momentum is being transferred?

[Radar]

Maybe I'm missing something, but couldn't energy, you know, be transferred to the surrounding air? As in, rather than being lost as heat, momentum is being transferred?

Sure, the disk pushes the air and this is why close to spinning objects you feel a slight breeze even if the object is a sphere or a disk so it kind of does not displace air directly.

So if you look at any given place close to the disk surface, the air is gaining momentum due to friction according to the local direction of disk velocity. If we consider just the spinning motion of a disk, what is the total velocity of the disk? 0. In turn the total momentum gained by air from the spinning motion is also 0 because at different parts of the disk the air is being pushed in different directions and those contributions cancel out. The transferred angular momentum is nonzero on the other hand and this is why the rotation slows down eventually.

It is a similar situation with the forward motion: a moving disk pushes the air in front of it and in this case as most of the air is pushed in one direction, there is momentum transferred from the disk to air. On the other hand from just a forward motion there will be no angular momentum gained by surrounding air.

This is just a detailed description of how the energy of the disk along with its momentum and angular momentum are dissipated. There is however no way for that energy to be transferred back from air to the disk.

[kyoryu]

Can I ask where you've seen this behavior?

[halfeye]

Can I ask where you've seen this behavior?

Mostly a long time ago, when I was playing with flying disks (Wikipedia calls them Frisbees, but I'm pretty sure there can be other brands).

[kyoryu]

I play disc golf. I see a lot of discs fly.

I haven't seen what you're describing. Here's what I do see, all of the time:

When discs fly fast, the generate more lift on the left. When they slow down, they generate more lift on the right.

When a disc flies fast enough, this will make it appear to turn to the right (really it's leaning to the right and its lift pushes it to the right). As it slows down, the opposite happens.

This can easily make the apparent velocity of the disc increase, as it deflects away from going "straight away". Also as it ends up on an angle, it will gain some downward velocity as gravity pulls it down.

[halfeye]

I play disc golf. I see a lot of discs fly.

I haven't seen what you're describing. Here's what I do see, all of the time:

When discs fly fast, the generate more lift on the left. When they slow down, they generate more lift on the right.

When a disc flies fast enough, this will make it appear to turn to the right (really it's leaning to the right and its lift pushes it to the right). As it slows down, the opposite happens.

This can easily make the apparent velocity of the disc increase, as it deflects away from going "straight away". Also as it ends up on an angle, it will gain some downward velocity as gravity pulls it down.

Are those different disks?

We were playing knockout (three touch and don't catch and you're out) and this was a deceptive throw, it was slow and looked easy to catch, but somehow you could judge the range right so it would wobble just as it came in reach. Fast throws could be difficult to catch, but in those young days usually weren't, but the wobbly ones were trickier, at least until you got used to them.

[kyoryu]

Are those different disks?

We were playing knockout (three touch and don't catch and you're out) and this was a deceptive throw, it was slow and looked easy to catch, but somehow you could judge the range right so it would wobble just as it came in reach. Fast throws could be difficult to catch, but in those young days usually weren't, but the wobbly ones were trickier, at least until you got used to them.

Yeah, but the basics of physics still apply. They're meant to fly faster without turning as much.

But if the disc is thrown a bit nose up, it can decelerate rapidly at the end and get that final left hand turn very quickly.

[halfeye]

Yeah, but the basics of physics still apply. They're meant to fly faster without turning as much.

But if the disc is thrown a bit nose up, it can decelerate rapidly at the end and get that final left hand turn very quickly.

Thrown as slowly as you can (it has to actually move, obviously), with as much spin as you can?

I think we were throwing flat, it wasn't far, even to the most distant person.

[kyoryu]

Thrown as slowly as you can (it has to actually move, obviously), with as much spin as you can?

I think we were throwing flat, it wasn't far, even to the most distant person.

When I talk about nose up, I don't mean the track of the disc - I'm talking about the angle of the disc. So you can throw flat but nose up (and it's really common).

[halfeye]

When I talk about nose up, I don't mean the track of the disc - I'm talking about the angle of the disc. So you can throw flat but nose up (and it's really common).

High spin and low speed is what matters. It will cross ten to fourty feet of distance before it slows to stalling, and it easily and reliably did that, it was an ordinary 10 inch approx disk.